Sign setter might be out of control? (6,2,3,3)

I believe the answer is:

beware of the dog

'sign' is the definition.

'setter might be out of control?' is the wordplay.
I cannot quite see how this works, but
'setter' could be 'dog' (a setter is a type of dog) and 'dog' is found within the answer.
'be' is found in the answer.
'of' could be 'o' and 'o' is found within the answer.
This explanation may well be incorrect...

I've seen this clue in The Independent.

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